HP 10C Series Technology and Packaging

HP began designing a new CPU called Saturn and series of calculators to follow
the HP-41C but these designs became quite complex so they designed another
series of midrange calculators based on improved HP-41C technology as a
short-term measure. The CPU for these new machines
included 61Kbits of ROM, 2.2Kbits of RAM, an LCD driver and a low-battery
detector on-chip. To improve on previous designs, HP developed a new CMOSC
process with design goals including:

Lower power consumption: An 85 thousand transistor circuit drew .25 milliwatts
and had a standby leakage of 5 - 10 nanoamperes. The process was meant to
allow calculators to run for a year from a set of small batteries but several
owners have reported that they are still running on their original batteries
after 20-22 years.

Higher density - three times that of previous ICs. This allowed sophisticated
calculators to be built with just two or three ICs and sell for modest prices.

High reliability: The ICs had intermediate oxide layers to trap contaminants
and were routinely inspected and tested at elevated temperatures. The ICs
were designed to survive electrostatic discharges in excess of 4000 volts.

Packaging

Previous HP's had a flexible sheet under the keyboard to protect the electronics
from minor spills and dirt. The 10C series took this further by having a
single large sheet of film under the keyboard that wrapped around the electronics
and overlapped at the back. This sheet consisted of three bonded layers:
the outer layers were nonconductive, but the center layer was conductive.
There was also an inner conductive film surrounding the main circuit board.
These films gave the CMOSC chips a high degree of electrostatic protection.

The majority of the calculator body housed only the keyboard. All of the
electronics where built onto a small flexible circuit film that was bonded
to the back of the LCD. A plastic fastener held the end of the circuit film
to the keyboard. The entire LCD/chip module sat on foam rubber pads (small
white squares seen in the picture above) to protect it when the calculator
was dropped.

A rubber sheet with a stainless steel snap disk layer attached to the lower
side sat between the protective film and the rigid keyboard circuit. The
disks connected the outer circle of the circuit board to the center point
when the keys were pressed. The top of the calculator was heatstaked to the
keyboard circuit board in 47 places to produce a rigid and consistent keyboard
feel.

The bottom half was pressed into the top half along slanted surfaces to make
a tight fit and attached with four screws which were then covered with the
rubber feet to produce a finished appearance. The result was a very small,
solid and durable calculator.